NAME
Filter::Template - a source filter for inline code templates (macros)
VERSION
version 1.043
SYNOPSIS
use Filter::Template;
# use Filter::Template ( isa => 'SomeModule' );
template max (one,two) {
((one) > (two) ? (one) : (two))
}
print {% max $one, $two %}, "\n";
const PI 3.14159265359
print "PI\n"; # Constants are expanded inside strings.
print "HAPPINESS\n"; # Also expanded due to naive parser.
enum ZERO ONE TWO
enum 12 TWELVE THIRTEEN FOURTEEN
enum + FIFTEEN SIXTEEN SEVENTEEN
# Prints numbers, due to naive parser.
print "ZERO ONE TWO TWELVE THIRTEEN FOURTEEN FIFTEEN SIXTEEN SEVENTEEN\n";
if ($expression) { # include
... lines of code ...
} # include
unless ($expression) { # include
... lines of code ...
} elsif ($expression) { # include
... lines of code ...
} else { # include
... lines of code ...
} # include
DESCRIPTION
Filter::Template is a Perl source filter that provides simple inline
source code templates. Inlined source code can be significantly faster
than subroutines, especially for small-scale functions like accessors
and mutators. On the other hand, they are more difficult to maintain and
use. Choose your trade-offs wisely.
Templates
Code templates are defined with the "template" statement, which looks a
lot like "sub". Because this is a naive source filter, however, the open
brace must be on the same line as the "template" keyword. Furthermore,
the first closing brace in column zero ends a macro body.
template oops {
die "Oops";
}
Templates are inserted into a program using a simple syntax that was
adapted from other template libraries. It was chosen to be compatible
with the Perl syntax highlighting of common text editors.
This inserts the body of "template oops".
{% oops %}
Templates can have parameters. The syntax for template parameters was
based on prototypes for Perl subroutines. The two main differences are
that parameters are named, and sigils are not used.
template sum_2 (parameter_0, parameter_1) {
print( parameter_0 + parameter_1, "\n" );
}
To insert a template with parameters, simply list the parameters after
the template name.
{% sum_2 $base, $increment %}
At expansion time, occurrences of the parameter names within the
template are replaced with the source code provided in the template
invocation. In the previous example, "sum_2" literally expands to
print( $base + $increment, "\n" );
and is then compiled by Perl.
Constants and Enumerations
Filter::Template also defines "const" and "enum" keywords. They are
essentially simplified templates without parameters.
"const" defines a constant that is replaced before compile time. Unlike
Perl's native constants, these are not demoted to function calls when
Perl is run in debugging or profiling mode.
const CONSTANT_NAME 'constant value'
const ANOTHER_CONSTANT 23
Enumerations are like constants but several sequential integers can be
defined in one statement. Enumerations start from zero by default:
enum ZEROTH FIRST SECOND
If the first parameter of an enumeration is a number, then the
enumerated constants will start with that value:
enum 10 TENTH ELEVENTH TWELFTH
Enumerations may not span lines, but they can be continued. If the first
enumeration parameter is the plus sign, then constants will start where
the previous enumeration left off.
enum 13 THIRTEENTH FOURTEENTH FIFTEENTH
enum + SIXTEENTH SEVENTEENTH EIGHTEENTH
Conditional Code Inclusion (#ifdef)
The preprocessor supports something like cpp's #if/#else/#endif by
usurping a bit of Perl's conditional syntax. The following conditional
statements will be evaluated at compile time if they are followed by the
comment "# include":
if (EXPRESSION) { # include
BLOCK;
} elsif (EXPRESSION) { # include
BLOCK;
} else { # include
BLOCK;
} # include
unless (EXPRESSION) { # include
BLOCK;
} # include
The code in each conditional statement's BLOCK will be included or
excluded in the compiled code depending on the outcome of its
EXPRESSION.
Conditional includes are nestable, but else and elsif must be on the
same line as the previous block's closing brace, as they are in the
previous example.
Filter::Template::UseBytes uses conditional code to define different
versions of a {% use_bytes %} macro depending whether the "bytes" pragma
exists.
IMPORTING TEMPLATES
Filter::Template can import templates defined by another class. For
example, this invocation imports the "use_bytes" template:
use Filter::Template ( isa => 'Filter::Template::UseBytes' );
Imported templates can be redefined in the current namespace.
Note: If the imported templates require additional Perl modules, any
code which imports them must also "use" those modules.
DEBUGGING
Filter::Template has three debugging constants which will only take
effect if they are defined before the module is first used.
To trace source filtering in general, and to see the resulting code and
operations performed on each line, define:
sub Filter::Template::DEBUG () { 1 }
To trace template invocations as they happen, define:
sub Filter::Template::DEBUG_INVOKE () { 1 }
To see template, constant, and enum definitions, define:
sub Filter::Template::DEBUG_DEFINE () { 1 }
To see warnings when a template or constant is redefined, define:
sub Filter::Template::DEFINE () { 1 }
CAVEATS
Source filters are line-based, and so is the template language. The only
constructs that may span lines are template definitions, and those must
span lines.
Filter::Template does not parse perl. The regular expressions that
detect and replace code are simplistic and may not do the right things
when parsing challenging Perl syntax. Constants are replaced within
strings, for example.
The regexp optimizer makes silly subexpressions like /(?:|m)/. That
could be done better as /m?/ or /(?:jklm)?/ if the literal is longer
than a single character.
The regexp optimizer does not optimize (?:x|y|z) as character classes.
The regexp optimizer is based on code in Ilya Zakharevich's Text::Trie.
Better regexp optimizers were released afterwards, and Filter::Template
should use one of them.
LINKS
BUG TRACKER
https://rt.cpan.org/Dist/Display.html?Status=Active&Queue=Filter-Templat
e
REPOSITORY
http://github.com/rcaputo/filter-template
http://gitorious.org/filter-template
OTHER RESOURCES
http://search.cpan.org/dist/Filter-Template/
SEE ALSO
Text::Trie, PAR, Filter::Template::UseBytes.
AUTHOR & COPYRIGHT
Filter::Template is Copyright 2000-2013 Rocco Caputo. Some parts are
Copyright 2001 Matt Cashner. All rights reserved. Filter::Template is
free software; you may redistribute it and/or modify it under the same
terms as Perl itself.
Filter::Template was previously known as POE::Preprocessor.